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Register a new userGreat Optically Luminous Dropout Research Using Subaru HSC (GOLDRUSH). I. UV Luminosity Functions at $z sim 4-7$ Derived with the Half-Million Dropouts on the 100 deg$^2$ Sky
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We study the UV luminosity functions (LFs) at $zsim 4$, $5$, $6,$ and $7$ based on the deep large-area optical images taken by the Hyper Suprime-Cam (HSC) Subaru strategic program (SSP). On the 100 deg$^2$ sky of the HSC SSP data available to date, we make enormous samples consisting of a total of 579,565 dropout candidates at $zsim 4-7$ by the standard color selection technique, 358 out of which are spectroscopically confirmed by our follow-up spectroscopy and other studies. We obtain UV LFs at $z sim 4-7$ that span a very wide UV luminosity range of $sim 0.002 - 100 , L_{rm UV}^ast$ ($-26 < M_{rm UV} < -14$ mag) by combining LFs from our program and the ultra-deep Hubble Space Telescope legacy surveys. We derive three parameters of the best-fit Schechter function, $phi^ast$, $M_{rm UV}^ast$, and $alpha$, of the UV LFs in the magnitude range where the AGN contribution is negligible, and find that $alpha$ and $phi^ast$ decrease from $zsim 4$ to $7$ with no significant evolution of $M_{rm UV}^ast$. Because our HSC SSP data bridge the LFs of galaxies and AGNs with great statistical accuracy, we carefully investigate the bright end of the galaxy UV LFs that are estimated by the subtraction of the AGN contribution either aided with spectroscopy or the best-fit AGN UV LFs. We find that the bright end of the galaxy UV LFs cannot be explained by the Schechter function fits at $> 2 sigma$ significance, and require either double power-law functions or modified Schechter functions that consider a magnification bias due to gravitational lensing.
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We present clustering properties from 579,492 Lyman break galaxies (LBGs) at z~4-6 over the 100 deg^2 sky (corresponding to a 1.4 Gpc^3 volume) identified in early data of the Hyper Suprime-Cam (HSC) Subaru strategic program survey. We derive angular correlation functions (ACFs) of the HSC LBGs with unprecedentedly high statistical accuracies at z~4-6, and compare them with the halo occupation distribution (HOD) models. We clearly identify significant ACF excesses in 10<$theta$<90, the transition scale between 1- and 2-halo terms, suggestive of the existence of the non-linear halo bias effect. Combining the HOD models and previous clustering measurements of faint LBGs at z~4-7, we investigate dark-matter halo mass (Mh) of the z~4-7 LBGs and its correlation with various physical properties including the star-formation rate (SFR), the stellar-to-halo mass ratio (SHMR), and the dark matter accretion rate (dotMh) over a wide-mass range of Mh/M$_odot$=4x10^10-4x10^12. We find that the SHMR increases from z~4 to 7 by a factor of ~4 at Mh~1x10^11 M$_odot$, while the SHMR shows no strong evolution in the similar redshift range at Mh~1x10^12 M$_odot$. Interestingly, we identify a tight relation of SFR/dotMh-Mh showing no significant evolution beyond 0.15 dex in this wide-mass range over z~4-7. This weak evolution suggests that the SFR/dotMh-Mh relation is a fundamental relation in high-redshift galaxy formation whose star formation activities are regulated by the dark matter mass assembly. Assuming this fundamental relation, we calculate the cosmic SFR densities (SFRDs) over z=0-10 (a.k.a. Madau-Lilly plot). The cosmic SFRD evolution based on the fundamental relation agrees with the one obtained by observations, suggesting that the cosmic SFRD increase from z~10 to 4-2 (decrease from z~4-2 to 0) is mainly driven by the increase of the halo abundance (the decrease of the accretion rate).
We investigate the dependence of galaxy clustering at $z sim 4 - 7$ on UV-luminosity and stellar mass. Our sample consists of $sim$ 10,000 Lyman-break galaxies (LBGs) in the XDF and CANDELS fields. As part of our analysis, the $M_star - M_{rm UV}$ relation is estimated for the sample, which is found to have a nearly linear slope of $dlog_{10} M_star / d M_{rm UV} sim 0.44$. We subsequently measure the angular correlation function and bias in different stellar mass and luminosity bins. We focus on comparing the clustering dependence on these two properties. While UV-luminosity is only related to recent starbursts of a galaxy, stellar mass reflects the integrated build-up of the whole star formation history, which should make it more tightly correlated with halo mass. Hence, the clustering segregation with stellar mass is expected to be larger than with luminosity. However, our measurements suggest that the segregation with luminosity is larger with $simeq 90%$ confidence (neglecting contributions from systematic errors). We compare this unexpected result with predictions from the textsc{Meraxes} semi-analytic galaxy formation model. Interestingly, the model reproduces the observed angular correlation functions, and also suggests stronger clustering segregation with luminosity. The comparison between our observations and the model provides evidence of multiple halo occupation in the small scale clustering.
We present the Ly$alpha$ luminosity functions (LFs) at $z=$5.7 and 6.6 derived from a new large sample of 1,266 Ly$alpha$ emitters (LAEs) identified in total areas of 14 and 21 deg$^2$, respectively, based on the early narrowband data of the Subaru/Hyper Suprime-Cam (HSC) survey. Together with careful Monte-Carlo simulations that account for the incompleteness of the LAE selection and the flux estimate systematics in the narrowband imaging, we have determined the Ly$alpha$ LFs with the unprecedentedly small statistical and systematic uncertainties in a wide Ly$alpha$ luminosity range of $10^{42.8-43.8}$ erg s$^{-1}$. We obtain the best-fit Schechter parameters of $L^{*}_{{rm Lya}}=1.6^{+2.2}_{-0.6} (1.7^{+0.3}_{-0.7}) times10^{43}$ erg s$^{-1}$, $phi^{*}_{{rm Lya}}=0.85^{+1.87}_{-0.77} (0.47^{+1.44}_{-0.44})times10^{-4}$ Mpc$^{-3}$, and $alpha=-2.6^{+0.6}_{-0.4} (-2.5^{+0.5}_{-0.5})$ at $z=5.7$ ($6.6$). We confirm that our best-estimate Ly$alpha$ LFs are consistent with the majority of the previous studies, but find that our Ly$alpha$ LFs do not agree with the high number densities of LAEs recently claimed by Matthee/Santos et al.s studies that may overcorrect the incompleteness and the flux systematics. Our Ly$alpha$ LFs at $z=5.7$ and $6.6$ show an indication that the faint-end slope is very steep ($alpha simeq -2.5$), although it is also possible that the bright-end LF results are enhanced by systematic effects such as the contribution from AGNs, blended merging galaxies, and/or large ionized bubbles around bright LAEs. Comparing our Ly$alpha$ LF measurements with four independent reionization models, we estimate the neutral hydrogen fraction of the IGM to be $x_{rm HI}=0.3pm0.2$ at $z=$6.6 that is consistent with the small Thomson scattering optical depth obtained by Planck 2016.
We constrain the rest-frame FUV (1546AA), NUV (2345AA) and U-band (3690AA) luminosity functions (LFs) and luminosity densities (LDs) with unprecedented precision from $zsim0.2$ to $zsim3$ (FUV, NUV) and $zsim2$ (U-band). Our sample of over 4.3 million galaxies, selected from the CFHT Large Area $U$-band Deep Survey (CLAUDS) and HyperSuprime-Cam Subaru Strategic Program (HSC-SSP) data lets us probe the very faint regime (down to $M_mathrm{FUV},M_mathrm{NUV},M_mathrm{U} simeq -15$ at low redshift) while simultaneously detecting very rare galaxies at the bright end down to comoving densities $<10^{-5}$ Mpc$^{-3}$. Our FUV and NUV LFs are well fitted by single Schechter functions, with faint-end slopes that are very stable up to $zsim2$. We confirm, but self-consistently and with much better precision than previous studies, that the LDs at all three wavelengths increase rapidly with lookback time to $zsim1$, and then much more slowly at $1<z<2$--$3$. Evolution of the FUV and NUV LFs and LDs at $z<1$ is driven almost entirely by the fading of the characteristic magnitude, $M^star_{UV}$, while at $z>1$ it is due to the evolution of both $M^star_{UV}$ and the characteristic number density $phi^star_{UV}$. In contrast, the U-band LF has an excess of faint galaxies and is fitted with a double-Schechter form; $M^star_mathrm{U}$, both $phi^star_mathrm{U}$ components, and the bright-end slope evolve throughout $0.2<z<2$, while the faint-end slope is constant over at least the measurable $0.05<z<0.6$. We present tables of our Schechter parameters and LD measurements that can be used for testing theoretical galaxy evolution models and forecasting future observations.
We perform a super-resolution analysis of the Subaru Hyper Suprime-Cam (HSC) images to estimate the major merger fractions of z~4-7 dropout galaxies at the bright end of galaxy UV luminosity functions (LFs). Our super-resolution technique improves the spatial resolution of the ground-based HSC images, from ~1 to <~0.1, which is comparable to that of the Hubble Space Telescope, allowing us to identify z~4-7 bright major mergers at a high completeness value of >~90%. We apply the super-resolution technique to 6535 very bright dropout galaxies in a UV luminosity range of L_UV~3-15 L_UV* corresponding to -24<~M_UV<~-22. The major merger fractions are estimated to be f_merger~5-20% at z~4 and ~50-80% at z~5-7, which shows no f_merger difference compared to those of a control faint galaxy sample. Based on the f_merger estimates, we verify contributions of source blending effects and major mergers to the bright-end of double power-law (DPL) shape of z~4-7 galaxy UV LFs. While these two effects partly explain the DPL shape at L_UV~3-10 L_UV*, the DPL shape cannot be explained at the very bright end of L_UV>~10 L_UV*, even after the AGN contribution is subtracted. The results support scenarios in which other additional mechanisms such as insignificant mass quenching effects and the low dust obscuration contribute to the DPL shape of galaxy UV LFs.
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